1. Technical Field
The present invention relates to portable computers in general, and in particular to a method for adjusting the load of a wireless base station. More particularly, the present invention relates to a method for allowing a wireless portable computer to adjust the load of a wireless base station autonomously.
2. Description of Related Art
Wireless terminals such as notebook-type portable computers (laptop PCs) or personal digital assistants (PDAs) have a wireless LAN communication device mounted therein and are capable of communicating with a server or other laptop PCs through a wireless base station. Moreover, in a wireless LAN network, multiple wireless base stations is disposed so that the ranges of radio coverage partially overlap with each other. The wireless terminal has a fallback function that automatically decreases a transmission rate so that the communication is not disconnected in order to perform signal processing of noise separated from the signals when the radio field intensity is weak. Thus, when multiple wireless base stations is detected to be connectable, the wireless terminal is configured to automatically connect to a wireless base station which provides the highest radio field intensity and from which a high transmission rate can be expected. Therefore, in a building where the wireless LAN network is installed, when many users carrying their wireless terminals gather in a certain space such as a meeting room, the connection is concentrated on a specific wireless base station which provides a strong radio field.
The number of wireless terminals connectable to one wireless base station is generally limited by the number of IP addresses assigned through the wireless base station. Although the IP addresses are automatically assigned by a DHCP server, a network administrator limits the number of IP addresses handled by a specific wireless base station from the perspective of managing security or the like. Therefore, the number of host addresses that are actually handled by a wireless base station is far smaller than the theoretical number defined in the global address class. Moreover, in some cases, the network administrator limits the number of wireless terminals connected to a specific wireless base station at the same time in order to maintain throughput. Since the IEEE 802.11 standards have no regulations on the time when roaming or disconnection occurs, a wireless terminal having connected to a specific wireless base station generally tends to maintain the connection unless it moves or is suspended. As a result, a number of wireless terminals which do not actually transmit data are connected to a specific wireless base station. Therefore, a wireless terminal which needs to transmit data is unable to connect to the specific wireless base station.
Moreover, even when multiple wireless terminals is connected to a specific wireless base station, only one wireless terminal can actually communicate with the wireless base station at a certain point in time. In order to avoid a collision of frames, the other wireless terminals perform carrier sensing and wait until a wireless channel enters an idle state and then transmit data. Therefore, if the number of wireless terminals connected to a specific wireless base station increases or the transmission of data from the respective wireless terminals is concentrated at a certain point in time, the overall transmission rate of all the wireless terminals connected to the wireless base station decreases. In addition, in the case of a collision of frames, the wireless terminal is unable to receive an ACK frame representing an acknowledgement from the wireless base station, the wireless terminal has to transmit the same data again, which further decreases the transmission rate.
As described above, when a number of wireless terminals connected to a specific wireless base station perform data transmission at the same time, the traffic (unit: bps) which is the quantity of data transmitted per unit period by the respective wireless terminals decreases. Thus, it becomes impossible to perform comfortable wireless communication. In a wireless base station, the throughput (unit: bps) which represents the quantity of data transmitted per unit period is set. For the respective wireless terminal to maintain a predetermined range of transmission rates, it is preferable to adjust the total traffic so that the throughput of the wireless base station has a predetermined margin with respect to the total traffic of the respective wireless terminal.